The well-known TESI (Transfer of Electrostatic Image) system of photoreproduction utilizes the creation of a latent, electrostatic image on a dielectric surface. This image comprises zones of electric charge selectively laid down on the surface of the dielectric sheet. After production of the electrostatic image, the electrostatic image surface is brought into contact with a toner material such as carbon black in combination with an adhesive resin, under such conditions that the toner material adheres to the electrostatically-charged portions of the dielectric surface, and does not adhere to the remaining portions. Accordingly, a visible image is formed on the dielectric surface.
A large amount of research has been invested into the TESI process, as well as other processes involving the creation of an electrostatic latent image on a dielectric surface. Examples of patents relating to this area of technology include: Carlson, et al. U.S. Pat. No. 2,982,647, Schaffert U.S. Pat. No. 3,147,679, Robinson U.S. Pat. No. 3,598,579, and the prior art of record in those patents.
In the TESI process, light is shined through a negative onto a photoconductive electrode. The light-transmitting portions of the negative permit activation of the light-irradiated portions of the photoconductive electrode, causing its electrical resistance to decrease. As a result of this, electrical current can selectively flow through the irradiated portions of the electrode across a small air gap of micronic size, to charge portions of the dielectric surface directly adjacent to the conducting portions of the photoconductive electrode.
However, the TESI process has not achieved notable commercial success, due to certain drawbacks, one of which is that the prior art systems have not been always capable of providing good, clear copies of negatives which exhibit relatively low contrast, such as diazo negatives, vesicular negatives and some silver negatives.
A need therefore exists to improve the TESI process, as well as other processes involving the creation of an electrostatic latent image, by enhancing the contrast of latent images. This can be done by increasing the electric charge density in the charged areas of the electrostatic latent images, relative to the uncharged areas of the image. When this is done, and toner is applied to the image, improved visual contrast can be obtained.